Efficient decontamination of Cr(III)-citrate complex with negligible Cr(VI) accumulation by pyrite-activated peroxymonosulfate process

The Cr(III)-organic complexes are recalcitrant substance that resist to most conventional methods due to high solubility, extremely slow ligand exchange rate, and the possible accumulation of more toxic Cr(VI) during treatment. In this study, we proposed a highly efficient and environmentally friendly PMS/pyrite-precipitation process for Cr(III)-citrate removal. Initial pH, pyrite dosage, peroxymonosulfate (PMS) concentration, and the coexisting anions (e.g., SO42-, Cl-, and NO3-) affected the removal rate of Cr(III)-citrate. Under optimal conditions (i.e., initial pH of 6, 4 g/L pyrite and 5 mM PMS), the combined process remove 98.3 +/- 1.5% total Cr and 62.9 +/- 0.8% total organic carbon (TOC) in 180 min, resulting in negligible Cr(VI) (< 0.01 mg/L). Pyrite-activated PMS generated SO4 center dot- and (OH)-O-center dot, which transform Cr(III)-citrate to Cr(VI), simultaneously releasing the organic ligands for further mineralization. SO4 center dot- is determined to contribute more in oxidative decomplexation. The generated Cr(VI) was reduced to Cr(III) rapidly by aqueous Fe(II) and structural Fe(II) in pyrite. Subsequently, the free metal ions and partial organic carbon were removed efficiently via alkaline precipitation. In addition, high removal efficiency of Cr(III)-citrate is maintained in the system after five consecutive cycles. This study suggests that the combined PMS/pyrite-precipitation process has great potential for efficient removal of Cr(III)-citrate from contaminated waters.